The present invention relates to a new and improved construction of a hygrometer or hygrostat.
Especially determinative of the response characteristic of hygrometers is the material from which its hygroscopic body is formed.
There are known to the art hygrometers whose hygroscopic body is formed of a hydrophilic plastic, especially a polyamide. With increasing ambient moisture this hygroscopic body lengthens or swells and with decreasing ambient moisture contracts or shortens. The selection of different polyamides, which possess these properties which are desirable for hygrometers, is comparatively limited.
The requirements placed upon hygrometers, namely, when such are employed for controlling watering or irrigation installations, are therefore so multifarious -- due to the requirements to be fulfilled -- that the existing selection of suitable plastics is not adequate in order to satisfactorily fulfill such requirements. With the heretofore known hygrometers there always is present a compromise between the properties offered by the plastics and the requirements placed upon the hygrometers.
For instance, the hygrometer firstly must be capable of responding with a considerable time-delay, even when the numerical value of the ambient moisture has reached the response value. Also, the hygrometer is confronted with the requirement that its response characteristic should not change linearly with the moisture content and/or the temperature of the surroundings, or that its response characteristic should possess hysteresis as a function of the time and/or as a function of the moisture content. With the prior art hygrometers it is therefore not possible to fulfill all of these requirements without resorting to the use of special regulation measures.